You have to pick:

I am unlikely to succeed at walking a tightrope across Niagra falls

At first I thought the bungee jump was the obvious safe bet, but then I realized that if the bungee rigging isn't properly set up I will smash to death on the side of the building. Whereas I could cross the tightrope quite safely by hanging under it from my hands and knees. So it depends.

I'm assuming when I bungee jump off the skyscraper that they have set up a proper bungee jumping platform which extrudes from the building. If not it would almost certainly be fatal.

I'd bungee jump. I love the idea of falling from great heights.

Bungee jumping off a skyscraper actually sounds like a lot of fun...

&%#$ im terrified of heights

for a seacond i thought niagra was narnia